1. Field of the Invention
This invention relates to means for controlling the movement and position of a platen on which a wafer may be mounted for use in process equipment such as that associated with ion implantation.
2. Description of the Prior Art
The conventional means for moving a platen uses air cylinders which drive gears to position the platen. The typical conventional means is illustrated in FIGS. 1A-C. The platen 15 is moved and positioned by the operation of the air cylinders 12, 13 which drive gears 20, 11. Gear 20 is cooperatively engaged with gear 11 and is fixably rotated by air cylinders 12, 13 by an interconnecting arm member 14. Interconnecting arm member 14 extends from air cylinder 13 and attaches to gear 20 by way of a shaft 17 and a clamp 19 (not shown). Platen 15 is attached to a shaft 16 by a clamp 18 (not shown) and is responsive to the cooperative movement of gears 20, 11. Air cylinders 12, 13 are connected in series to produce three fixed positions A-C. FIGS. 1A-C illustrate the three positions A-C of the series connected air cylinders. When cylinder 12 is in a full retract position while cylinder 13 is in a full extend position, as illustrated in FIG. 1A, the platen position is a wafer load position A. A wafer is loaded from a load lock 21 by gravity feed onto the platen. The platen moves to an implant position, which is approximately 0.degree.-15.degree. from the perpendicular. As illustrated in FIG. 1B, both air cylinders 12, 13 move into a full retract position to effect an implant position B of the platen. When the implantation process is completed, air cylinders 12, 13 both move into a full extend position which moves the platen into a wafer dump position C, as illustrated in FIG. 1C. The wafer is released from the platen and into the exit load lock 22.
Air cylinders characteristically have jerky and unpredictable movement and therefore provide less controllable movement of the platen. As a result, the cooperative movement of the air cylinders 12, 13 and gears 20, 11 do not control the movement and the positioning of platen 15 with the accuracy necessary for semiconductor processing, such as ion implantation. Positioning platen 15 is less predictable and inconsistent according to the conventional means because air cylinders 12, 13 do not stop at their full retract and full extend positions consistently. Moreover, clamp 19 which attaches air cylinder 13 to gear 20 and clamp 18 which attaches platen 15 to gear 11 of the conventional means are not hard-fixed to the assembly, but instead are friction type clamps which are operatively attached and detached with the gears. Therefore, clamps 18, 19 do not provide consistency to the conventional means of positioning a platen. Also, metal shielding is placed around platen 15 making visual verification of the platen's position difficult. This shielding makes it extremely difficult to check and set the alignment of platen 15 in relation to air cylinders 12, 13 and the gear train with clamps 18, 19. FIG. 2 illustrates the clamp 19 and bushing 19-1 design of the conventional means. This design is used to make a system with extremely high resistivity with respect to ground in order to allow the capture of ion charge. As illustrated in FIG. 2, platen 15 is located in the chamber under high vacuum and is hidden behind ion and x-ray shielding (not shown). In order to adjust the platen position or work on the platen, the vacuum chamber must be vented to the atmosphere. Venting of the chamber requires approximately 20 minutes evacuation time, but repumping the chamber to vacuum takes about 4 hours. Therefore, platen positioning is accomplished according to a more tedious but faster method which requires at least experience. In order to check the dump (FIG. 1C) and load (FIG. 1A) positions of platen 15, cylinders 12, 13 are placed in their proper position and the platen 15 positions are visually checked with the aid of a wafer. The wafer's thickness is a conventional and arbitrary standard choosen as the gap width necessary to prevent wafers from hitting the platen or exit lock in FIGS. 1A and 1C, respectively. This gap worked well to estimate the positions of the platen 15 and the wafer helped visualize this gap width. The platen is adjusted by loosening shaft clamp 18 and visually and manually setting the implant position B with a protractor. At all times, the wafer yield during ion implantation is sensitive to the implant angle accuracy. Once the implant position is set, clamp 18 is tightened The dump and load positions C, A are adjusted with mechanical stop screws which limit the travel of the air cylinders. The mechanical stop screws have limited adjustment capability and as a result, the adjustments made are inaccurate. The wafers are subject to many defects including wafer breakage as a result of the limitations with the conventional means.